The present invention relates to damper apparatus for retarding motion of a movable body.
Energy absorbers, or motion dampers, of many different types have been proposed to retard travel of a movable body. For example, piston-cylinder arrangements are common in which piston movement is retarded by restricting the flow of fluid between front and rear ends of the piston. Dampers of this type usually involve close fits and tight tolerances and require fluid seals which are subject to leakage and wear. Moreover, during damping actions, energy is converted into heat which changes the viscosity of the fluid and thereby alters the damping characteristics of the apparatus. In addition, extreme cleanliness is required in order to prevent blockage of the fluid-conducting passages. In cases where a compressible fluid is utilized as the damping medium, the overall system stiffness changes during a piston stroke, thus altering the damping characteristics.
Another type of energy absorber functions by converting frictional sliding between loaded surfaces into heat. Automobile brakes are representative of this type of energy absorber. Devices of this nature are subject to rapid wear. The damping forces that are generated are proportional to the forces applied in a direction perpendicular to the sliding surface, and nearly twice the amount of force is required to initiate motion than to sustain motion. Another characteristic is that the friction force is independent of the velocity of the body being damped.
Also known are dampers often referred to as visco elastic or elastomeric energy absorbers. These devices dissipate energy by distorting semi-rigid rubber or plastic-like materials. An example of such a device is a rolamite damper described in Research Report No. SC-RR-67-656 entitled "Rolamite: A New Mechanical Design Concept" by the present inventor printed in October, 1967, at pp. 152-4 (report issued by Sandia Corp. of Albuquerque, New Mexico and available from Clearinghouse for Federal Scientific and Technical Information, National Bureau of Standards, U.S. Department of Commerce, Springfield, Virginia). In such a damper, a rolamite band includes a thin visco elastic layer sandwiched between a pair of metal sections. The elastic layer is subjected to shearing action to dampen forces.
Another type of visco elastic damper operates through controlled distortion of the cross section of an elastic layer. In one known embodiment of this principle, an elastic tube slides within a cage having a series of balls. Advancement of the tube through the cage causes the balls to travel down a cam surface into engagement with the tube. In this position the balls rotate in place and compress the tube to retard tube advancement.
Visco elastic dampers of the above-described types are less expensive and are less subject to dramatic failure than other commonly used kinds of dampers. On the other hand, if such dampers are stopped for substantial periods in mid-stroke, the elastic may be subject to creep-induced deformations. These visco elastic dampers are also affected by temperature in that they offer less damping as temperature increases. Heat generated internally by such dampers is poorly dissipated because good visco elastic materials are poor thermal conductors. Also, it would seem that these types of visco elastic dampers exhibit an upper limit of the amount of damping they can provide.
It is apparent that dampers heretofore known have provided useful and advantageous service but exhibit certain disadvantages which limit the extent of their usefulness.